1. Field of the Invention
This invention relates to viscosity index improver containing oil compositions, particularly lubricating oil compositions, exhibiting reduced haze, and to a process for preparing such compositions. In particular, this invention is directed to low-haze or substantially haze-free lubricating oil compositions and additive packages used in their formulation containing ethylene-propylene copolymer viscosity index improvers and a haze-reducing effective amount of an oil-soluble hydrocarbyl substituted succinic acid.
2. Description of the Prior Art
An important property of a lubricating composition is the rate at which its viscosity changes as a function of temperature. The relationship between the viscosity and temperature is commonly expressed as the viscosity index (V.I.) Lubricant compositions which change little in viscosity with variations in temperature have a greater viscosity index than do compositions whose viscosity is materially affected by changes in temperature. One of the major requirements of the lubricating oils is a satisfactory viscosity-temperature characteristic so that the oils will not lose their fluidity but will show an equally good performance within a relatively wide temperature range to which they may be exposed in service.
In addition to refining natural petroleum oils to improve their viscosity index characteristics, it has been common practice to introduce long chain hydrocarbon compounds such as linear polymers in order to raise the viscosity index of lubricant compositions. Among the V.I. improvers that have been described in the patent literature are polyisobutylenes as taught in U.S. Pat. Nos. 2,084,501 and 2,779,753; polyalkylmethacrylates as described in U.S. Pat. No. 3,607,749; copolymers of alkylmethacrylates and styrene as shown in U.S. Pat. No. 3,775,329; hydrogenated butadienestyrene copolymers as shown in U.S. Pat. No. 2,798,853; and copolymers of butadiene, styrene and isoprene as shown in U.S. Pat. No. 3,795,615. It is also known to utilize ethylene-alpha-olefin copolymers such as ethylene-propylene copolymers as viscosity index improvers. A particularly useful type of ethylene-alpha-olefin copolymers are described in U.S. patent application Ser. No. 72,825, filed July 13, 1987. These copolymers are segmented copolymers of ethylene and at least one other alpha-olefin monomer; each copolymer is intramolecularly heterogeneous and intermolecularly homogeneous and at least one segment of the copolymer, constituting at least 10% of the copolymer's chain, is a crystallizable segment. The term "crystallizable segment" is defined to be each segment of the copolymer chain having a number-average molecular weight of at least 700 wherein the ethylene content is at least 55 wt. %. The remaining segments of the copolymer chain are herein termed the "low crystallinity segments" and are characterized by an average ethylene content of not greater than about 53 wt. %. Furthermore, the molecular weight distribution (MWD) of copolymer is very narrow.
However, during the preparation, processing, and/or storage of these oil soluble ethylene-alpha-olefin copolymers a haze may develop in their oil concentrates or oil compositions (e.g., oil concentrates or final oil formulations containing the ethylene-alpha-olefin copolymers). The source of this haze does not appear to be the same as that haze resulting from incompatibility of the several additives in a lubricating oil additive concentrate or composition (see U.S. Pat. No. 3,897,353 wherein haze resulting from component incompatibility is overcome in a lubricating oil additive concentrate by blending an amorphous ethylene-propylene copolymer with an n-alkyl methacrylate containing polymer having a number average molecular weight between about 30,000 and about 120,000). Rather, it is believed that this haze is due to the presence of a wide variety of catalysts, metal weak acid salts, etc. which are used in or result from the by-product of the polymerization, finishing process, or other steps in the manufacture or finishing of the ethylene-alpha olefin copolymers of this invention or their oil concentrates. Thus, for example, a typical haze producing substance is calcium stearate having a particle size of from about 0.01 microns to about 15 microns, which calcium stearate is used in the finishing process of these ethylene-alpha-olefin copolymers useful as V.I. improvers. Generally, oil compositions such as oil concentrates containing these ethylene-alpha-olefin copolymer viscosity index improvers also contain a haze forming amount of these metal weak acid salts. These haze forming amounts are generally less than about 1 wt. % based on the total weight of the oil compositions.
It has now been surprisingly discovered that the haze of oil compositions containing these ethylene-alpha-olefin copolymer viscosity index improvers can be reduced or substantially eliminated by the addition thereto or treatment thereof with a hydrocarbyl substituted succinic acid, a weak acid containing two hydrogen dissociating moieties both of which have pKs above 2.5 (e.g., succinic acid has a pK.sub.1 of 4.16 and a pK.sub.2 of 5.61).